For the sake of brevity and convenience, the discussion of the prior art here will be generally limited to the ankle, and prior art ankle prostheses with later discussion of the knee; accordingly:
As is known to those skilled in the art, until quite recently, fusion was the primary mode of treatment for the most disabling conditions of the ankle joint. The reason for this was that fusion of the ankle produces much less disfigurement and loss of function than does fusion of any other major load-bearing joint. Thus, ankle fusion, even with its attendant loss of function, remains a therapeutically acceptable procedure. Nevertheless, fusion, by its very nature, should be considered redical surgery--the last possible alternative in joint reconstruction. If effective prosthetic surfaces can restore normal ankle function, then such an alternative is certainly more desirable than fusion.
The article by Pappas, Buechel and DePalma entitled "Cylindrical Total Ankle Joint Replacement" which appeared in Clinical Orthopaedics and Related Research, No. 118, July-August 1976, pp. 82-92, surveys the various ankle prostheses which have been developed to date. All such prostheses are metal-to-plastic articular surface replacement types which essentially rely on ligamentous control for stability. The fundamental differences in the various designs lie in the nature of the articulating surfaces. Two basic types are found: those with theoretical line or point contact articulating surfaces and those with area contact articulating surfaces.
The incongruent surface types all permit the normally observed axial rotation in addition to permitting flexion-extension. However, incongruent surface prostheses suffer from two basic defects: (1) relatively poor wear and poor deformation resistance due to high local stress resulting from incongruent surface contact, and (2) relatively poor inherent stability. High local stresses and pressures resulting from normal walking loads, even with a relatively small amount of incongruity, result in permanent deformation of the ultra-high molecular weight polyethylene (UHMWPE) used in all known current ankle prostheses, as well as a relatively high wear rate.
In addition, since the human ankle joint possesses essentially congruent surfaces, one cannot expect an incongruent replacement joint to approximate normal motion since the kinematic properties of congruent and incongruent surfaces are so different.
Prostheses utilizing congruent or area contact surfaces, on the other hand, have good pressure distribution and, thus, offer superior wear and surface deformation resistance when compared to incongruent types. Further, they provide nearly normal stability because, under compressive load, the surfaces are forced into conformity and therefore motion to a large extent is defined by the surface geometry, thus providing predictable motion characteristics. Four basic variations of the area contact surface prosthesis are known. These are (1) spherical (e.g. ball and socket); (2) spheroidal (e.g. barrel-shaped); (3) cylindrical; and (4) conical.
The spherical prosthesis allows three independent axes of rotation but the joint surface geometry dictated by such a design tends to limit the flexion-extension range. Further, the spherical type ankle prosthesis is also less resistant to inversion-eversion injuries caused by substantially greater than normal loading of the ligaments and also is unstable since it allows an inversion-eversion motion between the tibia and talus that is not normally present. This lack of stability has been observed clinically.
The spheroidal type of prosthesis provides two independent rotations, plantar and dorsiflexion and inversion-eversion but fails to provide axial rotation. Thus, spheriodal prostheses are undesirable since axial rotation is necessary for normal function and since the device is also unstable in the inversion-eversion mode.
The conical type of prosthesis employs dual cones with a single horizontal axis. This design has an ample range of motion but it obtains this motion by the use of a substantially higher than normal rotation axis. Furthermore, this particular prosthesis requires significantly greater resection of bone than some other designs.
The cylindrical prosthesis, described in the above referred to article by Pappas et al. overcomes many of the noted deficiencies of the other congruent surface devices. Briefly, the device described by Pappas et al. employs a cylindrical surface with a horizontal axis located at the center of curvature of the lateral border of the talar dome. The cylindrical surface uses a UHMWPE talar component and a mortised cobalt-chromium alloy tibial component both of which are stabilized with methyl-methacrylate bone-cement and dual fixation fins.
As previously mentioned, the advantages of the cylindrical or conical configurations are internal stability, approximating that of the normal ankle, and good congruent contact producing low stress and wear. Unfortunately, a conventional cylindrical or conical joint does not provide for the axial rotation which is normally observed in ankle motion.
Strictly speaking, this rotation is not needed to provide a normal gait since it is not present at all in about 3 to 5% of normal individuals. However, in most individuals, the loading pattern and walk are such that axial rotation tends to be induced and if it is not provided for in the prosthesis, stress is imparted to the fixation means and on those components of the prosthesis which resist axial rotation, thereby introducing the possibility of loosening of the prosthesis or deformation or wear of the prosthesis.
More specifically, deformation and wear associated with the need for axial rotation have been found in a prosthesis that was removed from a patient after loosening of the prosthesis was encountered. It is believed that a major factor in this loosening was the failure of this early prosthesis to provide any degree of axial rotation.
Furthermore, it is known when heavy loads are impressed on a relatively thin plastic prosthesis the component tends to loosen. The explanation for this is that since a relatively weak and brittle substance is used to cement the plastic component to bone, when the plastic is deformed under load it easily bends transmitting this bending to the cementing material which is then subject to cracking, producing a loosening of the prosthesis. On the other hand, when this same brittle cement is used to fixture metal to bone, the metal is so rigid that bending is inhibited and therefore the bending stresses which tend to produce cracking on the cement are reduced to a level where they are no longer of concern. Thus the metal protects the cement.
Thus it is important that a prosthesis for a typical condylar type joint possess axial rotation and be fixtured to bone by use of a strong rigid material such as metal. This statement is supported by extensive clinical studies involving hundreds of patients with knee replacements which show that it is the plastic component, that most frequently fails by loosening or excessive deformation and that although these failures commonly occur in incongruent designs they are much more frequent in congruent designs which restrain axial rotation.
In view of the above, it is clearly desirable to affix a metal rather than plastic prosthesis to bone. Because metal-to-metal contact surfaces do not wear well and since the wear products cause adverse tissue reaction it is necessary to interpose a non-metallic bearing into the prosthesis.
The instant invention is based on the discovery that since it appears necessary in any event to interpose a third part in the prosthesis, if this part is implemented as a non-metallic insert, designed in such a manner that it provides the desired axial rotation, the previously discussed objectives will be attained. Futhermore, axial rotation of the prosthesis also compensates for surgical malalignment. That is, the components of the prosthesis inserted into the ankle can be slightly rotated. The ability of the insert to rotate axially accommodates for this rotational error.